Open-loop controls and closed-loop controls in injection moulding processes as a rule comprise the injection phase, during which a cavity in a tool is filled with a melt, the holding phase, which is to offset any material shrinkage, and a following cooling phase, during which the mass finally solidifies into the desired moulded part until the tool is finally opened and the finished moulded part can be removed. In all these phases, open-loop controls and closed-loop controls are necessary which are to ensure that the finished moulded parts are of consistent, reproducible quality.
In another production process, the injection compression, the injection process is followed by a compression process. With the fluid injection technique or the projectile technique, however, moulded parts are produced with cavities. In these production processes, the injection process is followed by a further process step, during which the hot plastic melt is displaced by a fluid.
In all these and further processes, the closed-loop control and open-loop control has to ensure during the injection phase in particular that the cavity is optimally filled. During injection moulding for example it is to be completely filled but not overfilled. If a plurality of cavities is present, this obviously applies to all cavities simultaneously.
The filling quantity in the cavity, as far as that is concerned, depends on the injection velocity of the injection screw and on the temperature of the nozzle prior to the injecting into the cavity. This decisively determines the viscosity of the plastic melt, which has an effect on the flow behaviour. At the correct time, the filling process has to be finally stopped.
In DE 2358911 a method is described, which describes a closed-loop device of injection moulding machines based on pressure and temperature measurements. Based on temperature measurements at different points, the flow front velocity is determined, based on which a control valve is finally set for the next cycle.
From EP 897786 a method for controlling an injection moulding plant is likewise known. This likewise controls different control parameters for the next cycle based on pressure measurements.
All these methods are highly complex and require sensors, which continuously capture measurements, which have to be evaluated and processed.
There are additional methods, during which a pressure or temperature sensor can determine the reaching of a melt front. In WO 2010/017940 and in JP 63239011 pressure sensors are for example employed for this purpose, in WO 2006/000411, WO 02/081177 and in DE 2358911 one or a plurality of temperature sensors. In AT 328173 the flow front direction is determined with a sensor pair, wherein the sensors supply temperature curves or pressure curves. Here it must be observed that with this method the entire measuring curves have to be captured in each case and interpreted from case to case. During the temperature measurements, triggers must then be set which depend on the tool temperature and on the melting temperature. A temperature sensor can thus be employed as flow front detection sensor only after further adjustments by a trained person. On changing the tool temperature it is necessary to adapt the temperature threshold. Fixed temperature differences can also be preset, which have to be reached during the injection in order to indicate the reaching of the flow front. However, this also includes an interpretation and an evaluation since the received measurements have to be recalculated into a temperature curve and compared with one another before the arriving of the flow front can be indicated. A flow front detection by means of temperature sensors always requires an evaluation and/or adaptation to the tool temperature, which is highly involved.
As pressure sensors, tool interior pressure sensors are possible on the one hand which can be installed in a bore that is open towards the cavity flush with the front of the cavity wall. Since these have a high requirement on the surface conformity so as not to leave any impressions on the casting behind, these are very expensive. On the other hand there are so-called measuring dowels which are arranged set back from the cavity in the tool wall. Such a measuring dowel is clamped in its bore and its measurement signal has to be interpreted and evaluated in a number of pre-tests corresponding to the respective installation positions so that the reaching of the sensor front can be reliably determined. In addition, such measuring dowels are not substantially cheaper than the mentioned tool interior pressure sensors and thus still too expensive.
Both with pressure sensors as well as with temperature sensors, time-dependent measurement curves are always captured and evaluated. This requires not only high-quality sensors but also a sophisticated evaluation software, in which individual settings have to be carried out. This is expensive and complicated.
In CH 667843 an optical lightwave sensor is installed in the cavity surface, wherein on the opposite cavity surface a lightwave receiver is installed in order to determine a presence of the flow front upon absence of the signal. Disadvantageous in this arrangement is that two bores have to be guided into the cavity both of which have to be closed in conformity with the surface. This method is not suitable for transparent materials. In addition, it is far too complicated, unreliable, expensive and complex.
FIG. 2b shows the method according to the prior art: a conventional measuring sensor 21 is arranged in a tool 4 near or directly on the cavity 3. This measuring sensor 21 as a rule is a temperature or pressure sensor. A measurement line 25 connects this measuring sensor 21 with a measurement evaluation and interpretation device 22. The measurement line has to be specially configured depending on the type of measuring element used. In particular, these are thermowire lines, such as NiCr/Ni or corresponding substitute lines when temperature measurements are carried out, or highly insulated lines, when pressure measurements are carried out. Correspondingly, for optical measurements it would have to be light conductors. Since the tool as a rule is surrounded by a tool holder that is not shown, a connector that is not shown is necessary at the transition as a rule. This connector in turn is subject to the same requirements relating to materials or insulation values as the lines 25, which follow said connector. All this makes the measuring device more expensive in addition to the expensive measurement sensor 21. On the measurement evaluation and interpretation device 22, all captured measurements are converted, interpreted and analysed. Measurement signals from temperature signals of thermocouples have to be captured at a compensation point, on which in turn the temperature is measured, and interpreted by means of a table. Following an evaluation, for example after determining a predefined temperature jump or pressure increase, a signal is finally given to an open-loop or closed-loop control unit 10 via a control value line 26, which can now be a conventional copper line. The latter finally controls the further process of the injection moulding operation in an open-loop or closed-loop manner based on the arriving of this signal via line 26 as desired in that it issues an open-loop or closed-loop control command 24. The measurement evaluation and interpretation device 22 and the open-loop or closed-loop control unit can be jointly accommodated in a housing 27 comprising both devices, but which does not constitute any simplification.